1. Field of the Invention
The invention relates generally to spectrometers and, more particularly, to a spectroscopic apparatus that digitizes an analog signal of widely varying range by combining the outputs of two or more ADCs that simultaneously digitize differently scaled versions of the same analog signal to provide a combined set of digitizations that cover the range with enhanced sensitivity and accuracy.
2. Discussion of Related Art
Spectroscopy is the science of identifying a sample""s relative degree of transmission, absorption, or reflection over a range of radiation frequencies. Spectroscopy can involve various ranges of radiation, i.e. visible radiation, mid-infrared radiation, ultraviolet radiation, and so on. Everybody is familiar with visible radiation, or visible xe2x80x9clightxe2x80x9d. Spectroscopy, however, often involves other ranges of radiation. Chemists, for example, often use mid-infrared radiation to determine the molecular content of a xe2x80x9csamplexe2x80x9d because different molecules absorb different amounts of the frequencies contained in such radiation. Each molecular species has a spectral xe2x80x9cfingerprintxe2x80x9d in the mid-infrared.
An instrument known as a spectrometer (or spectrophotometer) enables spectroscopic analysis. Earlier xe2x80x9cdispersivexe2x80x9d spectrometers rotated a dispersing element (grating or prism) through an arc so that all wavelengths within a desired range are presented to a detector. The industry subsequently developed spectrometers that use an interferometer to create a composite signal called an interferogramxe2x80x94a signal containing all frequencies in the entire spectrumxe2x80x94and then analyze the magnitude of each particular frequencies in that composite signal using the relatively complicated but well known mathematics of the Fourier Transform. Such interferometer-based spectrometers are often called Fourier Transform Infrared spectrometers, or simply FTIR spectrometers.
FIG. 1 shows a typical FTIR spectrometer 100. The device shown includes an interferometer module 110, a detector 120, and suitable sampling electronics 130. The FTIR spectrometers made by the assignee of this patent application use Michelson interferometers, as do others.
FIG. 2 shows a typical Michelson interferometer 110 in more detail. The basic principles and general operation of Michelson interferometers are well known. Nonetheless, the detailed construction of an exemplary Michelson interferometer is set forth in U.S. Pat. No. 3,936,193, the entire disclosure of which is hereby incorporated by reference in its entirety.
As shown in FIG. 2, a Michelson interferometer generally comprises a beamsplitter BS (e.g. a half silvered mirror), a fixed mirror M1, and a movable mirror M2. Light from a source enters the interferometer at point E. The beamsplitter BS is placed at an angle of 45xc2x0 with respect to the incoming light E such that it splits the light into two components: An xe2x80x9cAxe2x80x9d component travels through the beamsplitter to the fixed mirror M1. A xe2x80x9cBxe2x80x9d component is refelected by the beamsplitter toward the movable mirror M2. Both beams ultimately recombine (A+B) on the other side of the beam splitter, where they travel through a sample, and ultimately are focused onto a detector.
If mirrors M1 and M2 where alway at the same distance from the beamsplitter, the relative intensities of the wavelengths in the resultant beam (A+B) would be equal to their relative intensities in the initial beam E. At other unequal distances, the difference in the length of paths A and B will cause phase displacements of the various wavelengths such that, at the varying positions of movable mirror M2, each wavelength will optically interfere with itself to a degree proportional to the phase displacement resulting from such movement.
The movable mirror may be positioned at a point where the two mirrors are at an equal distance from the beam splitter, a position known as the Zero Path Difference of ZPD point. At this point, none of the incoming radiation is attenuated by phase displacement and the outgoing beam is at maximum intensity and equal to the incoming beam. During operation, the movable mirror M2 is scanned back and forth across the ZPD point position, as shown, thereby creating the interferogram by variably attenuating the plurality of wavelengths within the incoming beam E throughout its travel. Alternatively, in a different type of Michelson interferometer, a refractive wedge is interposed between the mirror M2 and the beamsplitter and moved to accomplish an interferometric scan without actually moving mirror M2.
The interferometer-modified radiation is passed through a sample or reflected from a sample, where it is further modified, and finally focused on a light sensitive detector that generates a fluctuating electrical signal, or detector signal, that is proportional to the intensity of the incident radiation.
Analog-to-digital conversion is a vital part of signal treatment because the detector signal has a very large dynamic range (i.e. is comprised of both low-level signals and high-level signals) and because accurate digitization of the low-level signals is necessary for good sensitivity and accuracy. Traditionally, this has resulted in the use of high-dynamic-range digitizers, sometimes combined with gain-switching. However, these prior approaches result in certain other problems.
High-dynamic-range digitizers are expensive, and tend to go out of adjustment over time. In particular, the monotonicity of digitization often degrades with time, especially around the zero voltage point of bipolar digitizers. Unfortunately, the low-level signal portion of instrument signals is usually near this zero voltage point, so this degradation seriously affects instrument performance. In addition, practical circuits generally have some semi-coherent noise resulting from the digital circuitry portion of the digitizer. In traditional digitizers, this noise is especially harmful because it is neither perfectly coherent (in some instruments such as the Fourier-transform infrared spectrometer, velocity variations cause the digitization rate to jitter), nor is it completely random.
Gain-switching is a method for utilizing the full range of a single digitizer during the low-level signal portion of the instrument""s measurement, while preventing digitizer overflow during the high-level portion of the instrument""s measurement. Gain-switching depends on a switchable-gain amplifier which must switch gain and stabilize during the short interval between digitizations (typically 10 to 30 microseconds in the case of a spectrometer). The design and commercial production of such a switch-gain amplifier is very difficult, and actual commercial realizations exist but have limitations. Maintaining exact gain, phase coherence, low noise, and zero stability over time while supplying an adequately short settling time has proven to be a formidable task.
Accordingly, there remains a need for a spectroscopic method and apparatus of using inexpensive digitizers to digitize an analog detector signal of very large dynamic range, with increased sensitivity and accuracy over that range, without requiring expensive, high-dynamic-range digitizers and without requiring gain-switching.
In a first aspect, the invention may be regarded as a method of operating an FTIR spectrometer to digitize an analog detector signal comprising the steps of passing an IR source signal through an interferometer; receiving an analog detector signal corresponding to the IR source signal; processing the analog detector signal with a low gain circuit to produce a low gain analog signal; processing the analog detector signal with a high gain circuit to produce a high gain analog signal; digitizing the low gain analog signal with a first ADC converter to produce a set of low gain samples; digitizing the high gain analog signal with a second ADC converter to produce a set of high gain samples, some of the high gain samples being below a predetermined threshold and some of the high gain samples being above the predetermined threshold; and merging the in-range high gain samples with the low gain samples to produce a combined set of samples.
In a second aspect, the invention may be regarded as a method of operating an FTIR spectrometer to digitize an analog detector signal comprising the steps of passing an IR source signal through an interferometer; receiving an analog detector signal corresponding to the IR source signal; processing the analog detector signal with a low gain circuit to produce a low gain analog signal; processing the analog detector signal with a high gain circuit to produce a high gain analog signal; digitizing the low gain analog signal with a first ADC converter to produce a set of low gain samples; digitizing the high gain analog signal with a second ADC converter to produce a set of high gain samples, some of the high gain samples being below a predetermined threshold and some of the high gain samples being above the predetermined threshold; identifying a plurality of pairs of low-gain samples and high-gain samples where the high-gain sample are below the predetermined threshold; performing a least squares fit to normalize the high-gain samples to the low-gain samples according to the equation y=Ax+B where each y value is a high-gain sample and each x value is a low-gain sample; and merging the in-range high gain samples with the low gain samples to produce a combined set of samples by using: a low-gain sample when a corresponding high-gain sample exceeds the predetermined threshold; and a high-gain sample, normalized relative to the low-gain samples as (yxe2x88x92B)/A, when the high-gain sample is less than the predetermined threshold.
In yet another aspect, the invention may be regarded as an FTIR spectrometer adapted for analyzing a sample with high dynamic range comprising: an interferometer for creating an interferogram from a source of IR radiation; a detector that produces an analog detector signal corresponding to the IR source signal; a low-gain circuit receiving the analog detector signal to producing a low-gain signal; a high-gain circuit receiving the analog detector signal and producing a high gain signal; a first analog-to-digital converter for digitizing the low gain signal to produce a set of low gain samples; a second analog-to-digital converter for digitizing the high gain signal to produce a set of high gain samples where some of the high gain samples are below a predetermined threshold and some of the high gain samples are above the predetermined threshold; and means for merging the high gain samples that are below the predetermined threshold with the low gain samples to produce a combined set of samples.
In alternative embodiments, the low gain circuit operates by passing the signal through unchanged or by attenuating the output of the high gain circuit.